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All electrochemical batteries are characterized by an electrical behavior that depends on temperature, state of charge, and current. In the case of lithium-ion accumulators, the energetical behavior is moreover deeply marked by line effects, due to the porosity of both electrodes. This paper deals with investigations on an accurate energetical modeling of lithium-ion battery. It is shown in particular that electrode porosity can be taken into account by means of a diffusion impedance represented by a capacitive transmission line. An energetical model, which couples this line with a current independent capacitance, is proposed and characterized at constant temperature (20°C) over different state-of-charge intervals (5%) and for different currents. Reactant diffusion within the electrolyte and relaxation period after discharge are investigated as well. Validation tests carried out on a 6.8-Ah lithium-ion element are conclusive.